Electrical submersible pump assemblies (ESP) are commonly used to pump hydrocarbon well fluid from wells. A common type of ESP has a centrifugal pump with a large number of pump stages, each stage having an impeller and a diffuser. An electrical motor rotates a shaft assembly that drives the impellers. The motor is filled with a dielectric lubricant. A seal section or pressure equalizer has a movable element, such as a bag or bellows, that equalizes the lubricant pressure with the hydrostatic pressure of the well fluid. The seal section is usually mounted between file motor and the pump, and the drive shaft assembly extends through the motor.
Seal sections with bellows may employ an outer bellows and an inner bellows which has an inner end recessed with the outer bellows. The seal section has upper and lower end members with a chamber between for receiving the inner and outer bellows. The enter bellows has one fixed end secured to one of the end members; the inner bellows has one fixed end secured to the other of the end members. Each of the inner and outer bellows has a floating end that moves as the bellows extends and contracts. An interconnect member secures the floating ends of the inner and outer bellows to each other. The interconnect member is cup-shaped, having a side wall located within the interior of the outer bellows.
During filling of the seal section with lubricant, ideally substantially all of the air within the inner and outer bellows is evacuated by a vacuum pump before the lubricant is introduced. A vacuum may be applied to an evacuation passage in the lower end member, which causes the outer bellows to contract and the inner bellows to extend. As the enter bellows contracts, the lower end of the interconnect member will eventually abut the lower end member, which stops further contraction of the outer bellows. At this point, there will still be some air located in the annulus between the interconnect member and the outer bellows. Eliminating this trapped air can be difficult.